In the last decade, developments of organic materials having semiconductor properties and applications thereof have been studied actively. The scope of applications of organic semiconductors is extending consistently, including electromagnetic shielding films, capacitors, organic electroluminescent (EL) displays, organic thin-film transistors, solar cells, memory devices based on multiphoton absorption, or the like. Since first studied in 1980, the organic transistors are actively researched recently over the world. Since it is expected that electronic circuit substrates characterized by easy fabrication, low cost, good impact resistance and bendability or foldability will be essential in the future industry, the development of organic transistors capable of meeting these requirements is emerging as a very task.
Although the existing inorganic-based semiconductor materials ensure superior properties and reliabilities, the trend is shifting toward organic semiconductor materials because of difficulty in device fabrication. When compared with the inorganic semiconductor materials, the organic semiconductor materials allow easy device fabrication at low cost and achievement of superior properties through simple modification of structure.
Common organic semiconductor materials exhibit a big difference in the mobility of holes and electrons. It is known that, in most cases, the hole mobility is 10 to 1000 times higher than the electron mobility.
For an organic semiconductor material to be applicable to a transistor, it should have a charge mobility high enough to exhibit the device performance. One way of increasing the charge mobility is doping. Brown et al. of Philips have doped the organic semiconductors tetrathiafulvalene (THF) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) with tetracyanoquinodimethane (TCNQ) and poly(β′-dodecyl-oxy(-α,α′-α′,α″-)terthienyl) (polyDOT3), respectively, at various molar ratios and investigated the mobility. As a result, it was found out that, although the charge mobility increases with the doping concentration, the on/off ratio decreases due to the increase in conductivity (A. R. Brown, D. M. de Leeuw, E. E. Havinga, and A. Pomp, Syn. Met., 68, 65, 1994). Accordingly, development of an organic field-effect transistor having a molecular system is necessary wherein the mobility increases more than the charge concentration does upon doping.
Meanwhile, there have been efforts to use a combination of a single molecule material and a polymer as an organic semiconductor. However, the performance of the polymer organic semiconductor is lower than that of the single molecule organic semiconductor. This may be because the film formation should be carried out in solution due to the large molecular weight of the polymer material, as reflected by the poor performance of a device fabricated in solution as compared to one fabricated by deposition of organic single molecules.
Studies have been made on copolymers having various repeat units. In particular, attempts have been made to develop new materials capable of reducing the energy band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for use as organic electronic materials such as organic semiconductors. Specifically, donor-acceptor (D-A) alternating conjugated polymers, which possess high light-harvesting aptitudes and ambipolar charge transporting abilities, have attracted special interests. For instance, a copolymer with alternating electron-deficient acceptor benzothiadiazole (BTD) and electron-donating cyclopentadithiophene exhibited hole mobilities of up to 1.4 cm2/(V·s) in organic field-effect transistors (H. N. Tsao, et al., Adv. Mater., 2009, 21, 209).
Benzobis(thiadiazole) (BBT) is also a potent electron acceptor and polymer organic electronic materials including the same as part of repeat units are studied.